Iron oxide based magnetic nanocrystals have been widely used in a variety of biomedical applications such as diagnostic, magnetic resonance imaging (MRI) and in magnetically guided site specific drug delivery systems. Their use as MRI contrast agents and as an enhancer in hypothermia (heating of diseased tissue by application of an RF pulse) has been widely discussed in the literature. Recently, it has been found that dextran coated iron oxide nanoparticles, ranging in size from 1 to 100 nm, can be used as magnetic relaxation switches (MRS) or magnetic relaxation nanosensors (MRnS). When these nanosensors self assemble in the presence of a molecular target, there is a significant change in the spin spin relaxation time (T2) of neighboring water molecules. This parameter (T2) is a component of the MR signal. The observed target-induced self assembly of iron oxide based nanoparticles has been used as a sensitive detection method for various targets and reported in the literature.